Exhaust gas cleaner and method for cleaning same

ABSTRACT

Nitrogen oxides are efficiently removed from an exhaust gas containing nitrogen oxides and an excess amount of oxygen, by (i) disposing an exhaust gas cleaner in a flow path of the exhaust gas, the exhaust gas cleaner comprising a first catalyst comprising 0.2-15 weight % (on a metal basis) of at least one silver salt selected from the group consisting of silver halides, silver sulfate and silver phosphate supported on a porous inorganic oxide on an inlet side and a second catalyst comprising (a) 0.2-15 weight % (on a metal basis) of at least one silver salt selected from the group consisting of silver halides, silver sulfate and silver phosphate and (b) up to 2 weight % (on a metal basis) of copper or copper oxide supported on a porous inorganic oxide on an outlet side; (ii) introducing at least one reducing agent selected from hydrocarbons and oxygen-containing organic compounds into the exhaust gas on an upstream side of the exhaust gas cleaner; and (iii) bringing the exhaust gas into contact with the exhaust gas cleaner at a temperature of 200°-600° C.

This is a division of application Ser. No. 08/340,329, filed Nov. 14,1994 now allowed.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas cleaner for effectivelyremoving nitrogen oxides by reduction and optionally carbon monoxide andhydrocarbons by oxidation from an exhaust gas containing nitrogen oxidesand an excess amount of oxygen, and a method for cleaning an exhaust gaswith such an exhaust gas cleaner.

Various exhaust gases discharged from internal combustion engines suchas automobile engines, etc., combustion apparatuses installed infactories, home fun heaters, etc. contain nitrogen oxides such asnitrogen monoxide and nitrogen dioxide together with an excess amount ofoxygen. The term "containing an excess amount of oxygen" means that theoxygen content is larger than its stoichiometric amount relative tounburned components such as carbon monoxide, hydrogen, hydrocarbons inthe exhaust gas. The term "nitrogen oxides" (NOx) means nitrogenmonoxide and/or nitrogen dioxide.

The nitrogen oxides are one of causes of acid rain, posing a seriousproblem of environmental pollution. For these reasons, various methodshave been proposed to remove nitrogen oxides from exhaust gases emittedfrom various combustion equipment.

In the case of large, stationary combustion apparatuses such as largecombustion apparatuses of factories, ammonia is introduced into anexhaust gas, whereby nitrogen oxides in the exhaust gas arecatalytically and selectively reduced (a selective catalytic reduction).

However, such a method is disadvantageous, because ammonia is expensive,because ammonia is so toxic that the amount of ammonia should becontrolled by measuring the concentration of nitrogen oxides in theexhaust gas, and because this reduction system generally needs largeapparatuses.

There is an alternative method for reducing NOx, which comprisescontacting an exhaust gas containing oxygen and NOx with a gaseousreducing agent such as hydrogen, carbon monoxide or hydrocarbons (anon-selective catalytic reduction). However, this method requires alarger amount of the reducing agent than its stoichiometric amountrelative to oxygen in the exhaust gas to carry out effective removal ofNOx. Accordingly, this method is effective only for the exhaust gashaving a relatively low oxygen concentration, which is generated byburning nearly at a theoretical air-fuel ratio.

There have been proposed methods of reducing nitrogen oxides by addingto an exhaust gas hydrocarbons in a smaller amount than a stoichiometricamount relative to oxygen in the exhaust gas, in the presence of acatalyst such as zeolite with or without carrying a transition metal(Japanese Patent Laid-Open Nos. 63-100919, 63-283727 and 1-130735;Thesis 2A526, 1990, the 59th Spring Conference of the Japan ChemicalSociety; Theses 3L420, 3L422 and 3L423, 1990, the 60th Fall Conferenceof the Japan Chemical Society; and "Catalyst", Vol. 33, No. 2, p.59(1991)).

However, these methods are effective only in a narrow temperature rangeof NOx removal. Also, their efficiency of removing nitrogen oxides isextremely low in the case of an actual exhaust gas because it containsabout 10% of moisture.

In addition, though it is important to remove unburned components suchas carbon monoxide, hydrocarbons, etc. from an exhaust gas, there hasnot been proposed a method capable of efficiently removing them togetherwith nitrogen oxides from the exhaust gas.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an exhaustgas cleaner and a method capable of efficiently removing nitrogen oxidesfrom an exhaust gas containing nitrogen oxides and oxygen in an amountlarger than the stoichiometric amount relative to unburned componentssuch as carbon monoxide, hydrogen, hydrocarbons, etc., which isdischarged from stationary combustion apparatuses, gasoline enginesoperated under oxygen excess conditions, and diesel engines.

Another object of the present invention is to provide an exhaust gascleaner and a method capable of efficiently removing nitrogen oxides byreduction and carbon monoxide and hydrocarbons by oxidation from anexhaust gas containing nitrogen oxides, unburned components such ascarbon monoxide, hydrogen, hydrocarbons, etc., and oxygen in an amountlarger than the stoichiometric amount relative to the unburnedcomponents, such an exhaust gas being discharged from stationarycombustion apparatuses, gasoline engines operated under oxygen excessconditions, and diesel engines.

As a result of intense research in view of the above objects, theinventors have found that nitrogen oxides are effectively removed evenfrom an exhaust gas containing about 10% of moisture with improvedefficiency in a wide temperature range, by using an exhaust gas cleanercomprising a silver salt such as silver halide, etc. supported on aporous inorganic oxide and Cu, Pt, Au, etc. supported on a porousinorganic oxide, and by adding a reducing agent such as hydrocarbons andoxygen-containing organic compounds to the exhaust gas and bringing theexhaust gas into contact with the exhaust gas cleaner in a particulartemperature range. The inventors have further found that nitrogenoxides, carbon monoxide and hydrocarbons are effectively removed evenfrom an exhaust gas containing about 10% of moisture with improvedefficiency in a wide temperature range, by using an exhaust gas cleanercomprising a silver salt such as silver halide, etc. supported on aporous inorganic oxide, copper or copper oxide supported on a porousinorganic oxide, and Pt, etc. supported on a porous inorganic oxide, andby adding a reducing agent such as hydrocarbons and oxygen-containingorganic compounds to the exhaust gas and bringing the exhaust gas intocontact with the exhaust gas cleaner in a particular temperature range.The present invention has been completed based on these findings.

Thus, a first exhaust gas cleaner for removing nitrogen oxides from anexhaust gas containing nitrogen oxides and oxygen in an amount largerthan its stoichiometric amount relative to unburned components in theexhaust gas according to the present invention comprises a firstcatalyst on an inlet side of the exhaust gas cleaner and a secondcatalyst on an outlet side of the exhaust gas cleaner, the firstcatalyst comprising 0.2-15 weight % (on a metal basis) of at least onesilver salt supported on a porous inorganic oxide, and the secondcatalyst comprising (a) 0.2-15 weight % (on a metal basis) of at leastone silver salt and (b) up to 2 weight % (on a metal basis) of copper orcopper oxide supported on a porous inorganic oxide.

A first method for removing nitrogen oxides from an exhaust gascontaining nitrogen oxides and oxygen in an amount larger than itsstoichiometric amount relative to unburned components in the exhaust gasaccording to the present invention comprises (i) disposing an exhaustgas cleaner in a flow path of the exhaust gas, the exhaust gas cleanercomprising a first catalyst on an inlet side of the exhaust gas cleanerand a second catalyst on an outlet side of the exhaust gas cleaner, thefirst catalyst comprising 0.2-15 weight % (on a metal basis) of at leastone silver salt supported on a porous inorganic oxide, and the secondcatalyst comprising (a) 0.2-15 weight % (on a metal basis) of at leastone silver salt and (b) up to 2 weight % (on a metal basis) of copper orcopper oxide supported on a porous inorganic oxide; (ii) introducing atleast one reducing agent selected from hydrocarbons andoxygen-containing organic compounds into the exhaust gas on an upstreamside of the exhaust gas cleaner; and (iii) bringing the exhaust gas intocontact with the exhaust gas cleaner at a temperature of 200°-600° C.,thereby causing a reaction of the nitrogen oxides with the reducingagent to remove the nitrogen oxides.

A second exhaust gas cleaner for cleaning an exhaust gas containingnitrogen oxides, unburned components comprising carbon monoxide andhydrocarbons, and oxygen in an amount larger than its stoichiometricamount relative to the unburned components, by removing nitrogen oxidesby reduction and carbon monoxide and hydrocarbons by oxidation from theexhaust gas according to the present invention comprises first, secondand third catalysts in this order from an inlet side to an outlet sideof the exhaust gas cleaner, the first catalyst comprising 0.2-15 weight% (on a metal basis) of at least one silver salt supported on a porousinorganic oxide, the second catalyst comprising (a) 0.2-15 weight % (ona metal basis) of at least one silver salt and (b) up to 2 weight % (ona metal basis) of copper or copper oxide supported on a porous inorganicoxide, and the third catalyst comprising up to 2 weight % (on a metalbasis) of at least one element selected from the group consisting of Pt,Pd, Ru, Rh, Ir and Au supported on a porous inorganic oxide.

A second method for cleaning an exhaust gas containing nitrogen oxides,unburned components comprising carbon monoxide and hydrocarbons, andoxygen in an amount larger than its stoichiometric amount relative tothe unburned components, by removing nitrogen oxides by reduction andcarbon monoxide and hydrocarbons by oxidation from the exhaust gasaccording to the present invention comprises (i) disposing an exhaustgas cleaner in a flow path of the exhaust gas, the exhaust gas cleanercomprising first, second and third catalysts in this order from an inletside to an outlet side of the exhaust gas cleaner, the first catalystcomprising 0.2-15 weight % (on a metal basis) of at least one silversalt supported on a porous inorganic oxide, the second catalystcomprising (a) 0.2-15 weight % (on a metal basis) of at least one silversalt and (b) up to 2 weight % (on a metal basis) of copper or copperoxide supported on a porous inorganic oxide, and the third catalystcomprising up to 2 weight % (on a metal basis) of at least one elementselected from the group consisting of Pt, Pd, Ru, Rh, Ir and Ausupported on a porous inorganic oxide; (ii) introducing at least onereducing agent selected from hydrocarbons and oxygen-containing organiccompounds into the exhaust gas on an upstream side of the exhaust gascleaner; and (iii) bringing the exhaust gas into contact with theexhaust gas cleaner at a temperature of 200°-600° C., thereby causing areaction of the nitrogen oxides with the reducing agent to removenitrogen oxides, carbon monoxide and hydrocarbons.

A third exhaust gas cleaner for removing nitrogen oxides from an exhaustgas containing nitrogen oxides and oxygen in an amount larger than itsstoichiometric amount relative to unburned components in the exhaust gasaccording to the present invention comprises a first catalyst comprising0.2-15 weight % (on a metal basis) of at least one silver salt having anaverage diameter of 10-1000 nm and supported on a porous inorganicoxide, and a second catalyst comprising 0.02-5 weight % (on a metalbasis) of at least one element selected from the group consisting of Pt,Pd, Ru, Rh, Ir and Au supported on a porous inorganic oxide.

A third method for removing nitrogen oxides from an exhaust gascontaining nitrogen oxides and oxygen in an amount larger than itsstoichiometric amount relative to unburned components in the exhaust gasaccording to the present invention comprises (i) disposing an exhaustgas cleaner in a flow path of the exhaust gas, the exhaust gas cleanercomprising a first catalyst comprising 0.2-15 weight % (on a metalbasis) of at least one silver salt having an average diameter of 10-1000nm and supported on a porous inorganic oxide, and a second catalystcomprising 0.02-5 weight % (on a metal basis) of at least one elementselected from the group consisting of Pt, Pd, Ru, Rh, Ir and Ausupported on a porous inorganic oxide; (ii) introducing at least onereducing agent selected from hydrocarbons-and oxygen-containing organiccompounds into the exhaust gas on an upstream side of the exhaust gascleaner; and (iii) bringing the exhaust gas into contact with theexhaust gas cleaner at a temperature of 150°-650° C., thereby causing areaction of the nitrogen oxides with the reducing agent to remove thenitrogen oxides.

A fourth method for removing nitrogen oxides from an exhaust gasdischarged from engines operated by using as a fuel mixed hydrocarbonsof liquefied petroleum gas, town gas or liquefied natural gas, theexhaust gas containing nitrogen oxides and oxygen in an amount largerthan its stoichiometric amount relative to unburned components in theexhaust gas according to the present invention comprises (i) disposingan exhaust gas cleaner in a flow path of the exhaust gas, the exhaustgas cleaner comprising a first catalyst comprising 0.2-15 weight % (on ametal basis) of at least one silver salt supported on a porous inorganicoxide, and a second catalyst comprising 0.02-5 weight % (on a metalbasis) of at least one element selected from the group consisting of Pt,Pd, Ru, Rh, Ir and Au supported on a porous inorganic oxide; (ii)introducing at least one hydrocarbon selected from the group consistingof liquefied petroleum gas, town gas, liquefied natural gas, methane andethane into the exhaust gas on an upstream side of the exhaust gascleaner; and (iii) bringing the exhaust gas into contact with theexhaust gas cleaner at a temperature of 150°-650° C., thereby causing areaction of the nitrogen oxides with the hydrocarbon to remove thenitrogen oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 1 and 2 andComparative Example 1;

FIG. 2 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 7 and 8 andComparative Example 4;

FIG. 3 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 9 and 10 andComparative Example 5;

FIG. 4 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 11-13 and ComparativeExample 6;

FIG. 5 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 14-18 and ComparativeExample 7; and

FIG. 6 is a graph showing the relation between the removal ratio of NOxand the temperature of the exhaust gas in Examples 19-23 and ComparativeExample 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides the following types of exhaust gascleaners and the corresponding methods for removing nitrogen oxides:

A! First catalyst: silver salt, and Second catalyst: silver salt andcopper or copper oxide.

B! First catalyst: silver salt, Second catalyst: silver salt and copperor copper oxide, and Third catalyst: Pt, Au, etc.

C! First catalyst: silver salt, and Second catalyst: Pt, Au, etc.

The nitrogen oxides are removed from an exhaust gas by bringing theexhaust gas into contact with the above exhaust gas cleaner, and byusing hydrocarbons remaining in the exhaust gas and/or hydrocarbonsand/or oxygen-containing organic compounds added to the exhaust gas onthe upstream side of the exhaust gas cleaner as reducing agents.

Each exhaust gas cleaner may be basically in two structures; one inwhich each of two or more catalysts is composed of a catalyticallyactive component carried by porous inorganic oxide powder, which is inturn supported on an exhaust gas cleaner substrate, and the other inwhich each of two or more catalysts is composed of a catalyticallyactive component directly carried by a porous inorganic oxide body.

In the former exhaust gas cleaner, the catalysts comprisingcatalytically active components supported on a porous inorganic oxidepowder are coated onto a heat-resistant exhaust gas cleaner substrate.Preferable materials for the exhaust gas cleaner substrate includeporous, heat-resistant ceramics such as γ-alumina, titania, zirconia andtheir composite oxides such as γ-alumina-titania, γ-alumina-silica,γ-alumina-zirconia, titania-zirconia, etc. When a high heat resistanceis required, cordierite, mullire, alumina or its composite oxides arepreferable. Also, the exhaust gas cleaner substrate may be formed from aknown metal material.

The shape and size of the exhaust gas cleaner substrate may be changeddepending on applications. Practically, it is preferable to form theexhaust gas cleaner substrate by a combination of two parts or more.Specifically, in the case of two parts, there are an inlet portion andan outlet portion. Also, in the case of three parts, there are an inletportion, an intermediate portion and an outlet portion. The exhaust gascleaner substrate may be in the form of a three-dimensional structuresuch as a honeycomb, a foam, a refractory fiber assembly, etc.

In the latter exhaust gas cleaner, the catalytically active componentsare supported directly by porous inorganic oxide bodies in the form ofpellets or granules, and they are charged into a proper reactor such asa catalytic converter.

A! First type of exhaust gas cleaner and method of removing NOx by usingsuch exhaust gas cleaner

The first type of exhaust gas cleaner comprises a first catalystcomprising 0.2-15 weight % (on a metal basis) of at least one silversalt supported on a porous inorganic oxide, and a second catalystcomprising (a) 0.2-15 weight % (on a metal basis) of at least one silversalt and (b) up to 2 weight % (on a metal basis) of copper or copperoxide supported on a porous inorganic oxide. The first catalyst isdisposed on the inlet side of the exhaust gas cleaner, and the secondcatalyst is disposed on the outlet side of the exhaust gas cleaner.

1! First catalyst

The first catalyst is at least one silver salt supported on a porousinorganic oxide, which is disposed on the inlet side of the exhaust gascleaner. The silver salt may include silver halides, silver sulfate andsilver phosphate. Preferable silver salts are silver chloride and silversulfate, and more preferable silver salt is silver chloride. Preferablematerials for the porous inorganic oxide include ceramics such asalumina, silica, titania, zirconia and their composite oxides, etc.Particularly preferable materials for the porous inorganic oxide areγ-alumina or its composite oxide such as γ-alumina-titania,γ-alumina-silica, γ-alumina-zirconia, etc. With γ-alumina or itscomposite oxides, the hydrocarbons remaining in the exhaust gas and/orat least one reducing agent selected from hydrocarbons andoxygen-containing organic compounds added to the exhaust gas areefficiently reacted with the nitrogen oxides in the exhaust gas.

A specific surface area of the porous inorganic oxide is preferably 10m² /g or more. When the specific surface area is smaller than 10 m² /g,the catalytically active component supported on a porous inorganic oxidedoes not come into contact with the exhaust gas in a large contact area,failing to remove nitrogen oxides efficiently. A specific surface areaof the porous inorganic oxide is more preferably 30 m² /g or more.

The amount of the silver salt supported as a catalytically activecomponent by the porous inorganic oxide is 0.2-15 weight %, preferably0.5-10 weight % (on a metal basis) based on the porous inorganic oxide(100 weight %). When the amount of the silver salt is less than 0.2weight %, a removal ratio of nitrogen oxides is low. On the other hand,when the amount of the silver salt is more than 15 weight %,hydrocarbons are likely to be predominantly burned, resulting in adecrease in the removal ratio of nitrogen oxides.

The silver salt component may be carried by the porous inorganic oxideby known methods such as an impregnation method, a precipitation method,a kneading method, etc. In the case of the impregnation method, forexample, the porous inorganic oxide is impregnated with an aqueoussolution of silver nitrate, etc., dried at 50°-120° C., furtherimpregnated with an aqueous solution of ammonium halide to convert thesilver nitrate into silver halide, etc., dried at 50°-150° C., andcalcined while elevating a temperature stepwise from about 100° C. toabout 600° C. The calcination may be conducted in the air, in an oxygenor nitrogen atmosphere, or in a hydrogen stream. When the calcination isconducted in a nitrogen atmosphere or in a hydrogen stream, it ispreferable to finally conduct an oxidation treatment, more preferably inthe presence of a small amount of nitrogen oxide.

The average diameter of the silver salt component after calcination ispreferably 10-1000 nm. The smaller the average diameter of the silversalt component, the higher the reactivity of the first catalyst. Whenthe average diameter of the silver salt component is less than 10 nm,hydrocarbons and/or oxygen-containing organic compounds are likely to bepredominantly burned, resulting in a decrease in the removal ratio ofnitrogen oxides. On the other hand, when it is larger than 1000 nm, thereactivity of the silver salt component is low, resulting in a lowremoval ratio of nitrogen oxides. The average diameter of the silversalt component is more preferably 10-500 nm, and most preferably 10-200nm. Incidentally, the average diameter is determined arithmetically.

The first catalyst may be carried by the exhaust gas cleaner substrateby known methods such as a wash-coating method, a powder method, etc.The thickness of the first catalyst carried by the exhaust gas cleanersubstrate is preferably 300 μm or less, more preferably 200 μm or less,though it is usually restricted by the difference in a thermal expansioncoefficient between the first catalyst and the substrate. With thisthickness, it is possible to prevent the exhaust gas cleaner from beingbroken by a thermal shock, etc. during NOx-removing operations.

The amount of the first catalyst coated onto a surface of the exhaustgas cleaner substrate is preferably 20-250 g/liter, more preferably50-200 g/liter based on the exhaust gas cleaner substrate. When theamount of the first catalyst is less than 20 g/liter, a sufficientremoval ratio of nitrogen oxides cannot be achieved. On the other hand,even when the amount of the first catalyst is more than 250 g/liter,correspondingly higher removal efficiency cannot be obtained, onlyresulting in a higher loss of pressure.

2! Second catalyst

The second catalyst comprises at least one silver salt selected from thegroup consisting of silver halides, silver sulfate and silver phosphateand copper or copper oxide supported as catalytically active componentsby a porous inorganic oxide, which is disposed on the outlet side of theexhaust gas cleaner. As in the first catalyst, the preferable materialsfor the porous inorganic oxide include porous, heat-resistant ceramicssuch as γ-alumina, silica, titania, zirconia and their composite oxidessuch as γ-alumina-titania, γ-alumina-silica, γ-alumina-zirconia,titania-zirconia, etc. As in the first catalyst, a specific surface areaof the porous inorganic oxide is preferably 10 m² /g or more, morepreferably 30 m² /g or more.

The catalytic_ally active components of the second catalyst are amixture of (a) at least one silver salt selected from the groupconsisting of silver halides, silver sulfate and silver phosphate and(b) copper or copper oxide. The amount of the silver salt is 0.2-15weight %, preferably 0.5-10 weight %, (on a metal basis) based on theporous inorganic oxide. When the amount of the silver salt is less than0.2 weight % or exceeds 15 weight %, a removal ratio of nitrogen oxidesis low. The amount of copper or copper oxide is up to 2 weight %,preferably up to 1.5 weight %, (on a metal basis) based on the porousinorganic oxide. When it exceeds 2 weight %, a removal ratio of nitrogenoxides is lowered.

The total amount of the catalytically active components (a) and (b) inthe second catalyst is more than 0.2 weight % and not more than 17weight % (on a metal basis), preferably 0.5-15 weight % (on a metalbasis) based on the porous inorganic oxide. When the total amount of thecatalytically active components exceeds 17 weight %, hydrocarbons arelikely to be predominantly burned, resulting in a decrease in theremoval ratio of nitrogen oxides. On the other hand, when it is lowerthan 0.2 weight %, the removal ratio of nitrogen oxides is low in alow-temperature region.

The second catalyst may further contain (c) at least one alkali metalelement and at least one rare earth element. The alkali metal elementmay be Na, K or Cs, and the rare earth element may be La, Ce or Nd. AMisch metal, a mixture of two or more rare earth elements, may also beused. In a case where the alkali metal element and the rare earthelement are combinedly used in the second catalyst, the total amount ofthe alkali metal element and the rare earth element is preferably 2weight % or less (on a metal basis), more preferably 0.5-1.5 weight %(on a metal basis). Further, the amount of the alkali metal element ispreferably 1 weight % or less, more preferably 0.1-0.5 weight %, and theamount of the rare earth element is preferably 1 weight % or less, morepreferably 0.1-0.5 weight %.

The total amount of the catalytically active components (a), (b) and (c)in the second catalyst is more than 0.2 weight % and not more than 19weight % (on a metal basis), preferably 0.5-15 weight % (on a metalbasis) based on the porous inorganic oxide. When the total amount of thecatalytically active components is lower than 0.2 weight %, noremarkable effect is exerted, resulting in a decrease in the removalratio of nitrogen oxides. On the other hand, when it exceeds 19 weight%, hydrocarbons are predominantly burned to reduce the removal ratio ofnitrogen oxides.

The catalytically active components of the second catalyst may becarried by the porous inorganic oxide by known methods such as animpregnation method, a precipitation method, etc. In the case of theimpregnation method, after carrying silver halide, etc. in the samemethod as in the first catalyst, the porous inorganic oxide isimpregnated with an aqueous solution of copper nitrate, etc., dried at50°-150° C., and heated at a temperature elevating stepwise from 100° C.to 600° C. The heating may be conducted in the air or in a nitrogen orhydrogen stream as in the first catalyst. Incidentally, thecatalytically active components (b) and (c) supported on the porousinorganic oxide are expressed herein by metals per se for the sake ofsimplicity, though they may exist in the form of metal or oxide at anexhaust gas temperature.

The thickness of the second catalyst carried by the exhaust gas cleanersubstrate is preferably 200 μm or less, more preferably 100 μm or less.The amount of the second catalyst carried by the substrate is preferably20-250 g/liter, more preferably 50-200 g/liter based on the substrate.Incidentally, the second catalyst may be carried by the exhaust gascleaner substrate by known methods such as a wash-coating method, etc.

A weight ratio of the first catalyst to the second catalyst is 5:1 to1:5. When the weight ratio of the first catalyst to the second catalystis less than 1:5 (when the percentage of the first catalyst is toosmall), a sufficiently high removal ratio of nitrogen oxides cannot beachieved in a wide temperature range of 200°-600° C. On the other hand,when the weight ratio is higher than 5:1 (when the percentage of thefirst catalyst is too large), a high removal ratio of nitrogen oxidescannot be achieved at a temperature of 400° C. or lower. Namely, areaction of the reducing agents with nitrogen oxides does not proceedwell at a relatively low temperature. The more preferred weight ratio ofthe first catalyst to the second catalyst is 4:1 to 1:4.

3! Method for removing NOx by using exhaust gas cleaner (first type)

The exhaust gas cleaner is disposed in a flow path of the exhaust gas,with the first catalyst on the inlet side and the second catalyst on theoutlet side.

The exhaust gas usually contains residual hydrocarbons such as ethylene,propylene, etc. to some extent. Since the exhaust gas generally does notcontain sufficient amounts of residual hydrocarbons to reduce allnitrogen oxides in the exhaust gas, at least one reducing agent selectedfrom hydrocarbons and oxygen-containing organic compounds should beintroduced into the exhaust gas. A site for introducing the reducingagent is an upstream side of the exhaust gas cleaner.

The hydrocarbons which are introduced into the exhaust gas may begaseous or liquid in the normal state. The gaseous hydrocarbons may bealkanes, alkenes or alkynes having 2 or more carbon atoms, such aspropane, propylene, acetylene, etc., and the liquid hydrocarbons may bediesel oil, cetane, heptane, kerosene, etc. The preferredoxygen-containing organic compounds are alcohols such as ethanol,butanol, etc. These reducing agents can be introduced into the exhaustgas by a spray method, etc.

A weight ratio of the reducing agent introduced into the exhaust gas tonitrogen oxides existing in the exhaust gas is preferably 0.1-5. Whenthe weight ratio is less than 0.1, a removal ratio of nitrogen oxides islow. On the other hand, when the weight ratio is more than 5, fuelefficiency is low. The weight ratio is more preferably 0.2 to 4.

The reaction of the nitrogen oxides with the hydrocarbons and/or theoxygen-containing organic compounds is efficiently carried out bycontrolling a catalyst ratio which means a ratio of the amount of theabove-mentioned catalyst to the amount of the exhaust gas per a unittime (catalyst amount/(exhaust gas amount/unit time)). From a practicalpoint of view, the catalyst ratio is preferably 0.005 sec·g/ml or more,more preferably 0.006 sec·g/ml or more.

The exhaust gas passing through the exhaust gas cleaner in which thehydrocarbons and/or the oxygen-containing organic compounds are reactedwith the nitrogen oxides is kept at a temperature of 200°-600° C. Whenthe temperature of the exhaust gas is lower than 200° C., a reductionreaction of the nitrogen oxides cannot be sufficiently carried out. Onthe other hand, when the temperature of the exhaust gas is higher than600° C., the hydrocarbons and/or the oxygen-containing organic compoundsare burned, failing to reduce the nitrogen oxides effectively. Thepreferred temperature of the exhaust gas is 300°-500° C.

By the method using the exhaust gas cleaner (first type), nitrogenoxides can be efficiently removed from the exhaust gas at a widetemperature range of 200°-600° C. even though the exhaust gas containsabout 10% of moisture.

B! Second type of exhaust gas cleaner and method of removing NOx, etc.by using such exhaust gas cleaner

The second type of exhaust gas cleaner comprises a first catalystcomprising 0.2-15 weight % (on a metal basis) of at least one silversalt supported on a porous inorganic oxide, a second catalyst comprising(a) 0.2-15 weight % (on a metal basis) of at least one silver salt and(b) up to 2 weight % (on a metal basis) of copper or copper oxidesupported on a porous inorganic oxide, and a third catalyst comprisingup to 2 weight % (on a metal basis) of at least one element selectedfrom the group consisting of Pt, Pd, Ru, Rh, Ir and Au supported on aporous inorganic oxide. The first, second and third catalysts aredisposed in this order from the inlet side to the outlet side of theexhaust gas cleaner.

1! First catalyst

The first catalyst of the second type of the exhaust gas cleaner mayessentially be the same as that of the first type of the exhaust gascleaner. Specifically, the first catalyst is at least one silver saltwhich may be selected from the group consisting of silver halides,silver sulfate and silver phosphate supported on a porous inorganicoxide, which is disposed on the inlet side of the exhaust gas cleaner toremove mainly nitrogen oxides in a high-temperature range. The materialsfor the porous inorganic oxide may be the same as in the first catalystof the first type of the exhaust gas cleaner.

As in the first catalyst of the first type of the exhaust gas cleaner,the amount of the silver salt supported as a catalytically activecomponent by the porous inorganic oxide is 0.2-15 weight % (on a metalbasis), preferably 0.5-10 weight % (on a metal basis) based on theporous inorganic oxide (100 weight %).

The silver salt component may be carried by the porous inorganic oxideby the same method as in the first type of the exhaust gas cleaner. Theaverage diameter of the silver salt component supported on the porousinorganic oxide after calcination is preferably 10-1000 nm, morepreferably 10-500 nm, and most preferably 10-200 nm.

The thickness of the first catalyst carried by the exhaust gas cleanersubstrate is preferably 200 μm or less, more preferably 100 μm or less.The first catalyst may be carried by the exhaust gas cleaner substrateby known methods such as a wash-coating method, etc. The amount of thefirst catalyst coated onto a surface of the exhaust gas cleanersubstrate is preferably 20-250 g/liter, more preferably 50-200 g/literbased on the exhaust gas cleaner substrate.

2! Second catalyst

The second catalyst of the second type of the exhaust gas cleaner mayessentially be the same as that of the first type of the exhaust gascleaner. Specifically, the second catalyst comprises (a) at least onesilver salt selected from the group consisting of silver halides, silversulfate and silver phosphate and (b) copper or copper oxide supported ascatalytically active components by a porous inorganic oxide, which isdisposed downstream of the first catalyst to remove mainly nitrogenoxides in a low-temperature range. The materials for the porousinorganic oxide may be the same as in the second catalyst of the firsttype of the exhaust gas cleaner.

The amount of the silver salt is 0.2-15 weight %, preferably 0.5-10weight % (on a metal basis) based on the porous inorganic oxide. Theamount of copper or copper oxide is up to 2 weight %, preferably up to1.5 weight % (on a metal basis) based on the porous inorganic oxide. Thetotal amount of the catalytically active components (a) and (b) in thesecond catalyst is more than 0.2 weight % and not more than 17 weight %,preferably 0.5-15 weight % (on a metal basis) based on the porousinorganic oxide.

The second catalyst may further contain (c) 1 weight % or less,preferably 0.1-0.5 weight % of at least one alkali metal element such asNa, K or Cs and 1 weight % or less, preferably 0.1-0.5 weight % of atleast one rare earth element such as La, Ce or Nd. The total amount ofthe alkali metal element and the rare earth element may be preferably 2weight % or less (on a metal basis), more preferably 0.5-1.5 weight %(on a metal basis).

The total amount of the catalytically active components (a), (b) and (c)in the second catalyst is more than 0.2 weight % and not more than 19weight % (on a metal basis), preferably 0.5-15 weight % (on a metalbasis) based on the porous inorganic oxide.

The catalytically active components of the second catalyst may becarried by the porous inorganic oxide by the same method as in the firsttype of the exhaust gas cleaner.

The thickness of the second catalyst carried by the exhaust gas cleanersubstrate is preferably 200 μm or less, more preferably 100 μm or less.The amount of the second catalyst carried by the substrate is preferably20-250 g/liter, more preferably 50-200 g/liter based on the substrate.

A weight ratio of the first catalyst to the second catalyst ispreferably 5:1 to 1:5, more preferably 4:1 to 1:4.

3! Third catalyst

The third catalyst comprises at least one element selected from thegroup consisting of Pt, Pd, Ru, Rh, Ir and Au supported on a porousinorganic oxide, which is disposed on the outlet side of the exhaust gascleaner (downstream of the second catalyst). The third catalystfunctions to remove nitrogen oxides in a low-temperature range, and tooxidize carbon monoxide and hydrocarbons. Preferable materials for theporous inorganic oxide may be the same as in the first catalyst.

The amount of the catalytically active component of the third catalystis up to 2 weight % (on a metal basis), preferably 0.1-1.5 weight % (ona metal basis) based on the porous inorganic oxide. When the amount ofthe catalytically active component of the third catalyst exceeds 2weight %, hydrocarbons are likely to be predominantly burned, resultingin a decrease in the removal ratio of nitrogen oxides.

When gold is added as a component of the second catalyst with or withoutPt, etc., the amount of gold is 0.02-5 weight (on a metal basis) basedon the porous inorganic oxide. When the amount of gold is less than 0.02weight %, a sufficient removal ratio of nitrogen oxides cannot beachieved. On the other hand, when it exceeds 5 weight %, hydrocarbonsand/or oxygen-containing organic compounds are likely to bepredominantly burned, resulting in a decrease in the removal ratio ofnitrogen oxides. The preferred amount of gold is 0.02-2 weight % (on ametal basis) based on the porous inorganic oxide.

The third catalyst may further contain at least one rare earth elementsuch as La, Ce, etc. in an amount of 10 weight % or less (on a metalbasis) based on the porous inorganic oxide. With the rare earth elementsupported together, the third catalyst (platinum catalyst) is providedwith an improved heat resistance.

The catalytically active component of the third catalyst may be carriedby the porous inorganic oxide by known methods such as an impregnationmethod, a sol-gel method, etc. In the case of the impregnation method,the porous inorganic oxide is impregnated with an aqueous solution ofchlorides, nitrates, etc. of an element for the catalytically activecomponent, dried at 50°-150° C., and heated at a temperature elevatingstepwise from 100° C to 700° C.

The third catalyst may be carried by the exhaust gas cleaner substrateby known methods such as a wash-coating method, a sol-gel method, apowder method, etc. The thickness of the third catalyst carried by theexhaust gas cleaner substrate is preferably 200 μm or less, morepreferably 100 μm or less. The amount of the third catalyst carried bythe substrate is preferably 20-250 g/liter, more preferably 50-200g/liter based on the substrate.

A weight ratio of the first catalyst to the third catalyst is preferably5:1 to 1:5. When the weight ratio of the first catalyst to the thirdcatalyst is less than 1:5 (when the percentage of the first catalyst istoo small), a sufficiently high removal ratio of nitrogen oxides cannotbe achieved in a wide temperature range of 200°-600° C. On the otherhand, when the weight ratio is higher than 5:1 (when the percentage ofthe first catalyst is too large), high removal ratios of nitrogenoxides, carbon monoxide and hydrocarbons cannot be achieved at atemperature of 400° C. or lower. The more preferred weight ratio of thefirst catalyst to the third catalyst is 4:1 to 1:4.

4! Method for removing NOx by using exhaust gas cleaner (second type)

The exhaust gas cleaner is disposed in a flow path of the exhaust gas,with the first catalyst on the inlet side, the third catalyst on theoutlet side and the second catalyst interposed therebetween.

The hydrocarbons and the oxygen-containing organic compounds which areintroduced into the exhaust gas as the reducing agent may be the same asin the method using exhaust gas cleaner (first type). A weight ratio ofthe reducing agent to nitrogen oxides existing in the exhaust gas ispreferably 0.1-5, more preferably 0.2 to 4.

From a practical point of view, the catalyst ratio is preferably 0.005sec·g/ml or more, more preferably 0.006 sec·g/ml or more.

The exhaust gas passing through the exhaust gas cleaner is kept at atemperature of 200°-600° C., preferably 300°-500° C.

By the method using the exhaust gas cleaner (second type), nitrogenoxides can be efficiently removed from the exhaust gas at a widetemperature range of 200°-600° C. even though the exhaust gas containsabout 10% of moisture.

C! Third type of exhaust gas cleaner and method of removing NOx by usingsuch exhaust gas cleaner

The third type of exhaust gas cleaner comprises a first catalystcomprising 0.2-15 weight % (on a metal basis) of at least one silversalt selected from the group consisting of silver halides, silversulfate and silver phosphate having an average diameter of 10-1000 nmand supported on a porous inorganic oxide, and a second catalystcomprising 0.02-5 weight % (on a metal basis) of at least one elementselected from the group consisting of Pt, Pd, Ru, Rh, Ir and Ausupported on a porous inorganic oxide.

In one embodiment, the first catalyst is disposed on the inlet side ofthe exhaust gas cleaner, and the second catalyst is disposed on theoutlet side of the exhaust gas cleaner. Also, in another embodiment, amixture of the first catalyst and the second catalyst is used.

When Au is used together with Pt, etc., in one embodiment, the firstcatalyst is disposed on the inlet side of the exhaust gas cleaner, Au isdisposed in an intermediate portion of the exhaust gas cleaner, and thesecond catalyst (Pt, etc.) is disposed on the outlet side of the exhaustgas cleaner. In another embodiment, a mixture of the first catalyst, Auand the second catalyst (Pt, etc.) is used.

1! First catalyst

The first catalyst of the third type of the exhaust gas cleaner mayessentially be the same as that of the first type of the exhaust gascleaner. Specifically, the first catalyst is at least one silver saltselected from the group consisting of silver halides, silver sulfate andsilver phosphate supported on a porous inorganic oxide. The materialsfor the porous inorganic oxide may be the same as in the first catalystof the first type of the exhaust gas cleaner.

As in the first catalyst of the first type of the exhaust gas cleaner,the amount of the silver salt supported as a catalytically activecomponent by the porous inorganic oxide is 0.2-15 weight % (on a metalbasis), preferably 0.5-10 weight % (on a metal basis) based on theporous inorganic oxide (100 weight %).

The silver salt component may be carried by the porous inorganic oxideby the same method as in the first type of the exhaust gas cleaner. Theaverage diameter of the silver salt component supported on the porousinorganic oxide after calcination is preferably 10-1000 nm, morepreferably 10-500 nm, and most preferably 10-200 nm.

The thickness of the first catalyst carried by the exhaust gas cleanersubstrate is preferably 200 μm or less, more preferably 100 μm or less.The first catalyst may be carried by the exhaust gas cleaner substrateby known methods such as a wash-coating method, etc.

The amount of the first catalyst coated onto a surface of the exhaustgas cleaner substrate is preferably 20-250 g/liter, more preferably50-200 g/liter based on the exhaust gas cleaner substrate.

2! Second catalyst

The second catalyst may be the same as the third catalyst of the secondtype of the exhaust gas cleaner. Specifically, the catalytically activecomponent of the second catalyst is at least one element selected fromthe group consisting of Pt, Pd, Ru, Rh, Ir and Au, and preferablecombinations thereof are Pt+Rh, Pd+Rh, and Pt+Pd+Rh. The amount of thecatalytically active component of the second catalyst is 0.02-5 weight %(on a metal basis), preferably 0.05-2 weight % based on the porousinorganic oxide. When gold is added as a third component with or withoutPt, etc., the amount of gold is 0.02-5 weight %, preferably 0.05-1weight % (on a metal basis) based on the porous inorganic oxide.

The second catalyst may further contain at least one rare earth elementsuch as La, Ce, etc. in an amount of 10 weight % or less (on a metalbasis) based on the porous inorganic oxide. With the rare earth elementsupported together, the second catalyst (platinum catalyst) is providedwith an improved heat resistance.

When gold is added as a component of the second catalyst with or withoutPt, etc., the second catalyst may further contain at least one alkalineearth metal element such as Ca, Mg, etc., and at least one rare earthelement such as La, Ce, etc. The amount of the alkaline earth metalelement is preferably 2 weight or less, more preferably 0.01-1 weight %,and the amount of the rare earth element is preferably 2 weight % orless, more preferably 0.01-1 weight %.

The catalytically active component of the second catalyst may be carriedby the porous inorganic oxide by the same method as in the thirdcatalyst of the second type of the exhaust gas cleaner. When gold isused, it is preferably carried by a porous inorganic oxide such astitania, zinc oxide, magnesium oxide, alumina and composite oxidesthereof.

The second catalyst may be carried by the exhaust gas cleaner substrateby known methods such as a wash-coating method, a sol-gel method, apowder method, etc. The thickness of the second catalyst carried by theexhaust gas cleaner substrate is preferably 200 μm or less, morepreferably 100 μm or less. The amount of the second catalyst carried bythe substrate is preferably 20-250 g/liter, more preferably 50-200g/liter based on the substrate.

A weight ratio of the first catalyst to the second catalyst ispreferably 5:1 to 1:5, more preferably 4:1 to 1:4.

3! Method for removing NOx by using exhaust gas cleaner (third type)

In one embodiment, the exhaust gas cleaner (third type) is disposed in aflow path of the exhaust gas, with the first catalyst on the inlet sideand the second catalyst on the outlet side. In another embodiment, theexhaust gas cleaner comprising a mixture of the first catalyst and thesecond catalyst is disposed in a How path of the exhaust gas.

The hydrocarbons or the oxygen-containing organic compounds which areintroduced into the exhaust gas as the reducing agent in this method maybe the same as in the method using exhaust gas cleaner (second type).The weight ratio of the reducing agent to nitrogen oxides existing inthe exhaust gas is preferably 0.1-5, more preferably 0.2-4.

From a practical point of view, the catalyst ratio is preferably 0.005sec·g/ml or more, more preferably 0.006 sec·g/ml or more.

The exhaust gas passing through the exhaust gas cleaner is kept at atemperature of 150°-650° C. When the temperature of the exhaust gas islower than 150° C., a reduction reaction of the nitrogen oxides cannotbe sufficiently carried out. On the other hand, when the temperature ofthe exhaust gas is higher than 650° C., the hydrocarbons oroxygen-containing organic compounds are burned, failing to reduce thenitrogen oxides effectively. The preferred temperature of the exhaustgas is 300°-600° C.

4! Method for removing NOx from exhaust gas discharged from enginesoperated by liquefied petroleum gas, etc.

The third type of exhaust gas cleaner is also effective to removenitrogen oxides from an exhaust gas discharged from engines operated byusing as a fuel mixed hydrocarbons of liquefied petroleum gas, town gasor liquefied natural gas.

The third type of exhaust gas cleaner is disposed in a flow path of theexhaust gas with the first catalyst on the inlet side and the thirdcatalyst on the outlet side, or a mixture of the first catalyst and thethird catalyst is disposed in a flow path.

In this case, mixed hydrocarbons or hydrocarbons are introduced into theexhaust gas. The hydrocarbons may include alkanes, alkenes or alkynessuch as methane, ethane, propane, butane, etc., and the mixedhydrocarbons may include liquefied petroleum gas, town gas and liquefiednatural gas. Other mixed hydrocarbons may also be used. With the mixedhydrocarbons and the hydrocarbons, the removal ratio of NOx in ahigh-temperature range can be improved. When the mixed hydrocarbons areadded, it is preferable to use those having many carbon atoms sincesaturated hydrocarbons having a small number of carbon atoms serve todecrease the removal ratio of NOx in a low-temperature range.

A weight ratio of the mixed hydrocarbons or the hydrocarbons introducedinto the exhaust gas to nitrogen oxides existing in the exhaust gas is 5or less, preferably 0.1-5, more preferably 0.2-4.

The exhaust gas passing through the exhaust gas cleaner is kept at atemperature of 150°-650° C., preferably 300°-600° C.

By the method using the exhaust gas cleaner (third type) and using theoxygen-containing organic compounds, nitrogen oxides can be efficientlyremoved from the exhaust gas at a wide temperature range even though theexhaust gas contains about 10% of moisture.

Though the first to third types of the exhaust gas cleaners areexplained separately for the sake of avoiding confusion, it should benoted that materials and methods described with respect to a particulartype of exhaust gas cleaner may be utilized for other exhaust gascleaners unless they do not deviate from the spirit of the presentinvention. For instance, with respect to the details of catalysts andporous inorganic oxides, please refer to Section A!.

The present invention will be described in further detail by way of thefollowing Examples. Incidentally, the catalytic active components aregenerally expressed by metals themselves for simplicity in Examples.

EXAMPLE 1

Commercially available γ-alumina pellets (diameter: 1.5 mm, length: 6mm, specific surface area: 260 m² /g) were immersed in an aqueoussolution of silver nitrate (0.67 g of silver nitrate/20 ml of water) for20 minutes, dried at 80° C. for 2 hours, and then further dried at 180°C. for 2 hours in a dry nitrogen stream. After cooling to roomtemperature in a dry nitrogen stream, the γ-alumina pellets wereimmersed in an aqueous solution of ammonium chloride (0.5 g of ammoniumchloride/20 ml of water) for 12 hours to convert the silver nitrate onthe γ-alumina pellets into silver chloride. The γ-alumina pellets werethen dried at 80° C. for 2 hours in air, heated to 550° C. at a heatingrate of 2.5° C./min, and calcined at 550° C. for 5 hours in an nitrogenstream containing 10 % of oxygen to prepare a first catalyst(silver-salt catalyst) carrying 2 weight % (on a metal basis) of silverchloride.

Separately, γ-alumina pellets carrying thereon 2 weight (on a metalbasis) of silver chloride were prepared in the same manner. Theγ-alumina pellets thus prepared were then immersed in an aqueoussolution of copper nitrate, dried and then heated to 600° C. stepwise inthe air to prepare a second catalyst (silver-salt/copper catalyst)carrying 2 weight % (on a metal basis) of silver chloride and 0.07weight % (on a metal basis) of copper.

An exhaust gas cleaner consisting of 3.75 g of the first catalyst(silver-salt catalyst) and 3.75 g of the second catalyst(silver-salt/copper catalyst) was charged into a reactor pipe with thesilver salt catalyst on the inlet side and the silver-salt/coppercatalyst on the outlet side.

Next, a test gas having the composition shown in Table 1 below wascaused to pass through the reactor pipe at a rate of 4.4 liters perminute (normal state), which corresponded to an overall apparent spacevelocity of 15,000 h⁻¹ and catalyst ratios of the first and secondcatalysts of 0.05 sec·g/ml. The temperature of the test gas was kept at250° C. in the reactor pipe to cause a reaction between nitrogenmonoxide and propylene. The test was repeated while changing thetemperature of the test gas as shown in FIG. 1.

                  TABLE 1                                                         ______________________________________                                        Component       Concentration                                                 ______________________________________                                        Nitrogen monoxide                                                                             800 ppm                                                       Oxygen          10 volume %                                                   Propylene       1714 ppm                                                      Nitrogen        Balance                                                       Water           10 volume % of the total volume                                               of the above components                                       ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemihminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 1.

EXAMPLE 2

In the same manner as in Example 1, γ-alumina powder (average diameter:40 μm, specific surface area: 200 m² /g) carrying 3 weight % (on a metalbasis) of silver chloride was prepared. Then, 1.4 g of the resultingfirst catalyst (silver-salt catalyst) was supported on a commerciallyavailable honeycomb-type cordierire filter (diameter: 30 mm, length:12.5 mm) by a wash-coating method, dried and then heated to 600° C. toprepare an exhaust gas cleaner part carrying the silver-salt catalyst.

Further, in the same manner as in Example 1, γ-alumina powder (averagediameter: 40 μm, specific surface area: 200 m² /g) carrying thereon 3weight % (on a metal basis) of silver chloride and 0.09 weight % (on ametal basis) of copper was prepared (second catalyst) Then, 1.4 g of thesecond catalyst was supported on a honeycomb-type cordierire filter ofthe same type as above, dried and then heated to 600° C. stepwise toprepare an exhaust gas cleaner part carrying the silver-salt/coppercatalyst.

With the silver-salt catalyst on the inlet side and thesilver-salt/copper catalyst on the outlet side, the catalyst-carryinghoneycomb-type cordieritc filters were combined and charged in a reactorpipe. Under the same conditions as in Example 1, tests were conductedusing the test gas having the composition shown in Table 1. The resultsare shown in FIG. 1.

Comparative Example 1

In the same manner as in Example 1, an exhaust gas cleaner comprisingγ-alumina pellets carrying 2 weight % (on a metal basis) of silverchloride was prepared. 7.50 g of this exhaust gas cleaner was charged ina reactor pipe, and tests were conducted using the test gas having thecomposition shown in Table 1 under the same conditions as in Example 1.The results are shown in FIG. 1.

As is clear from FIG. 1, the exhaust gas cleaners in Examples 1 and 2which comprise both of the silver-salt catalyst and thesilver-salt/copper catalyst can provide a high removal ratio of nitrogenoxides in a wide temperature range of the exhaust gas. On the otherhand, the exhaust gas cleaner in Comparative Example 1 which comprisesonly the silver-salt catalyst shows a low removal ratio of nitrogenoxides in a low-temperature range of the exhaust gas.

EXAMPLE 3

A first catalyst (silver-salt catalyst) carrying 2 weight % (on a metalbasis) of silver chloride was prepared by carrying silver chloride oncommercially available γ-alumina pellets (diameter: 1.5 mm, length: 6mm, specific surface area: 200 m² /g) in the same manner as in Example1, drying and then heating them to 600° C. stepwise. A second catalyst(silver-salt/copper catalyst) carrying 2 weight % (on a metal basis) ofsilver chloride and 0.07 weight % (on a metal basis) of copper wasprepared by carrying silver chloride and copper on γ-alumina pellets(diameter: 1.5 mm, length: 6 mm, specific surface area: 200 m² /g) inthe same manner as in Example 1, drying and then heating them to 600° C.

Further, γ-alumina pellets (diameter: 1.5 mm, length: 6 mm, specificsurface area: 200 m² /g) were immersed in an aqueous solution ofchloroplatinic acid, dried and then heated to 700° C. to prepare a thirdcatalyst (platinum catalyst) carrying 0.2 weight % (on a metal basis) ofPt.

An exhaust gas cleaner consisting of 3.75 g of the first catalyst(silver-salt catalyst), 3.75 g of the second catalyst(silver-salt/copper catalyst) and 1.8 g of the third catalyst (platinumcatalyst) was charged into a reactor pipe with the silver-salt catalyston the inlet side, the platinum catalyst on the outlet side and thesilver-salt/copper catalyst interposed therebetween.

Next, a test gas having the composition shown in Table 2 below wascaused to pass through the reactor pipe at a rate of 4.4 liters perminute (normal state), which corresponded to an overall apparent spacevelocity of 12,000 h⁻¹ and catalyst ratios of the first and thirdcatalysts of 0.05 sec·g/ml and 0.025 sec·g/ml, respectively. Thetemperature of the test gas was kept at 250° C. in the reactor pipe tocause a reaction between nitrogen monoxide and propylene. The test wasrepeated while changing the temperature of the test gas as shown inTable 3.

                  TABLE 2                                                         ______________________________________                                        Component        Concentration                                                ______________________________________                                        Nitrogen monoxide                                                                              800 ppm                                                      Carbon monoxide  100 ppm                                                      Oxygen           10 volume %                                                  Propylene        1714 ppm (three times the                                                     weight of nitrogen monoxide)                                 Nitrogen         Balance                                                      Water            10 volume % of the total volume                                               of the above components                                      ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. Similarly, the removal ratios ofcarbon monoxide and hydrocarbons were determined. The results are shownin Table 3.

EXAMPLE 4

In the same manner as in Example 2, γ-alumina powder (specific surfacearea: 200 m² /g) carrying 2 weight % (on a metal basis) of silverchloride was prepared. Then, 1 g of the resulting first catalyst(silver-salt catalyst) was supported on a commercially availablehoneycomb-type cordieritc filter (diameter: 20 mm, length: 12.6 mm, 400cell/in²) in the same manner as in Example 2 to prepare an exhaust gascleaner part carrying the silver-salt catalyst.

Further, in the same manner as in Example 2, γ-alumina powder (specificsurface area: 200 m² /g) carrying thereon 2 weight (on a metal basis) ofsilver chloride and 0.07 weight % (on a metal basis) of copper wasprepared (second catalyst). Then, 1 g of the resulting second catalyst(silver-salt/copper catalyst) was supported on a honeycomb-typecordieritc filter (diameter: 20 mm, length: 12.6 mm) in the same manneras in Example 2 to prepare an exhaust gas cleaner part carrying thesilver-salt/copper catalyst.

Further, in the same manner as in Example 3, γ-alumina powder (specificsurface area: 200 m² /g) carrying thereon 0.2 weight % (on a metalbasis) of Pt and 0.2 weight % (on a metal basis) of Rh was prepared(third catalyst). Then, 0.5 g of the third catalyst was supported on ahoneycomb-type cordieritc filter (diameter: 20 mm, length: 6 mm) in thesame manner as in Example 2 to prepare an exhaust gas cleaner partcarrying the platinum/rhodium catalyst.

With the silver-salt catalyst on the inlet side, the silver-salt/coppercatalyst in an intermediate portion and the platinum/rhodium catalyst onthe outlet side, these exhaust gas cleaner parts were combined andcharged in a reactor pipe. Under the same conditions (overall apparentspace velocity: 12,000 h⁻¹)as in Example 3, tests were conducted usingthe test gas having the same composition as in Table 2 except forcontaining diesel oil in an amount three times that of nitrogen monoxideinstead of propylene. The results are shown in Table 3.

Comparative Example 2

In the same manner as in Example 3, an exhaust gas cleaner composed ofy-alumina pellets carrying 2 weight % (on a metal basis) of silverchloride was prepared. 7.50 g of this exhaust gas cleaner was charged ina reactor pipe, and tests were conducted using the test gas having thecomposition shown in Table 2 under the same conditions as in Example 3.The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Removal Ratios of Nitrogen Oxides,                                            Carbon Monoxide and Hydrocarbons                                              Temperature                                                                            Components                                                                              Removal Ratio (%)                                          (°C.)                                                                           Removed   Example 3 Example 4                                                                            Com. Ex. 2                                ______________________________________                                        250      NOx       30        25     0                                                  CO        75        78     40                                                 HC*       50        52     35                                        300      NOx       42        43     8                                                  CO        90        90     50                                                 HC*       65        70     40                                        400      NOx       64        54     35                                                 CO        98        100    60                                                 HC*       95        95     65                                        500      NOx       70        72     68                                                 CO        100       100    70                                                 HC*       100       100    75                                        550      NOx       74        78     72                                                 CO        100       100    75                                                 HC*       100       100    80                                        ______________________________________                                         Note *: Hydrocarbon.                                                     

As is clear from Table 3, the exhaust gas cleaners in Examples 3 and 4which comprise the first to third catalysts can provide high removalratios of nitrogen oxides, carbon monoxide and hydrocarbon in a widetemperature range of the exhaust gas. On the other hand, the exhaust gascleaner in Comparative Example 2 which comprises only the silver-saltcatalyst is effective for the removal of nitrogen oxides, carbonmonoxide and hydrocarbons in a narrow temperature range of the exhaustgas.

EXAMPLE 5

The exhaust gas cleaner prepared in Example 2 was charged into a reactorpipe with the silver-salt catalyst on the inlet side and thesilver-salt/copper catalyst on the outlet side. Next, a test gas havingthe composition shown in Table 4 below was caused to pass through thereactor pipe at an overall apparent space velocity of 15,000 h⁻¹. Thetemperature of the test gas was kept at 250° C. in the reactor pipe tocause a reaction between nitrogen monoxide and ethanol. The test wasrepeated while changing the temperature of the test gas as shown inTable 5.

                  TABLE 4                                                         ______________________________________                                        Component       Concentration                                                 ______________________________________                                        Nitrogen monoxide                                                                             800 ppm (dry base)                                            Carbon dioxide  10 volume % (dry base)                                        Oxygen          10 volume % (dry base)                                        Ethanol         three times the weight of                                                     nitrogen monoxide (dry base)                                  Nitrogen        Balance                                                       Water           10 volume % of the total volume                                               of the above components                                       ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in Table 5.

EXAMPLE 6

The exhaust gas cleaners prepared in Example 4 was charged into areactor pipe with the silver-salt catalyst on the inlet side, thesilver-salt/copper catalyst in an intermediate portion and theplatinum/rhodium catalyst on the outlet side. Under the same conditions(overall apparent space velocity: 12,000 h⁻¹) as in Example 5, testswere conducted using the test gas having the composition shown in Table4. The results are shown in Table 5.

Comparative Example 3

The exhaust gas cleaner prepared in Comparative Example 1 was chargedinto a reactor pipe in the same manner as in Comparative Example 1.Under the same conditions (overall apparent space velocity: about 30,000h⁻¹) as in Example 5, tests were conducted using the test gas having thecomposition shown in Table 4. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Removal Ratios of Nitrogen Oxides                                             Temperature                                                                             Removal Ratio %                                                     (°C.)                                                                            Example 5    Example 6                                                                              Com. Ex. 3                                    ______________________________________                                        250       68.0         72.0     30.0                                          300       85.8         89.5     40.2                                          350       90.2         93.0     60.8                                          400       70.0         68.0     72.1                                          450       58.0         55.5     75.5                                          500       50.3         45.4     62.1                                          550       45.4         40.0     55.4                                          ______________________________________                                    

As is clear from Table 5, the exhaust gas cleaners in Examples 5 and 6can provide high removal ratios of nitrogen oxides in a wide temperaturerange of the exhaust gas, especially, in a low-temperature range of theexhaust gas, as compared with the exhaust gas cleaner in ComparativeExample 3.

EXAMPLE 7

Commercially available γ-alumina pellets (diameter: 1.5 mm, length: 6mm, specific surface area: 200 m² /g) were immersed in an aqueoussolution of silver nitrate (0.67 g of silver nitrate/20 ml of water) for20 minutes, dried at 80° C. for 2 hours, and then further dried at 180°C. for 2 hours in a dry nitrogen stream. After cooling to roomtemperature in a dry nitrogen stream, the γ-alumina pellets wereimmersed in an aqueous solution of ammonium chloride (0.5 g of ammoniumchloride/20 ml of water) for 12 hours to convert the silver nitrate onthe γ-alumina pellets into silver chloride. The γ-alumina pellets werethen dried at 80° C. for 2 hours in air, heated to 600° C. at a heatingrate of 2.5° C./min, and calcined at 600° C. for 5 hours in an nitrogenstream containing 10% of oxygen to prepare a first catalyst (silver-saltcatalyst) carrying 3 weight % (on a metal basis) of silver chloridehaving an average diameter of 45 nm.

Further, γ-alumina pellets (diameter: 1.5 mm, length: 6 mm, specificsurface area: 200 m² /g) were immersed in an aqueous solution ofchloroplatinic acid, dried and then heated to 700° C. to prepare asecond catalyst (platinum catalyst) carrying 3 weight % (on a metalbasis) of Pt.

An exhaust gas cleaner consisting of 3.7 g of the first catalyst(silver-salt catalyst) and 1.8 g of the second catalyst (platinumcatalyst) was charged into a reactor pipe with the silver-salt catalyston the inlet side and the platinum catalyst on the outlet side.

Next, a test gas having the composition shown in Table 6 below wascaused to pass through the reactor pipe at a rate of 4.4 liters perminute (normal state), which corresponded to an overall apparent spacevelocity of 20,000 h⁻¹ and catalyst ratios of the first and secondcatalysts of about 0.05 sec·g/ml and about 0.025 sec·g/ml, respectively.The temperature of the test gas was kept at about 250° C. in the reactorpipe to cause a reaction between nitrogen monoxide and propylene. Thetest was repeated while changing the temperature of the test gas asshown in FIG. 2.

                  TABLE 6                                                         ______________________________________                                        Component       Concentration                                                 ______________________________________                                        Nitrogen monoxide                                                                             800 ppm                                                       Carbon monoxide 100 ppm                                                       Oxygen          10 volume %                                                   Propylene       1714 ppm                                                      Nitrogen        Balance                                                       Water           10 volume % of the total volume                                               of the above components                                       ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 2.

EXAMPLE 8

In the same manner as in Example 7, γ-alumina powder (specific surfacearea: 200 m² /g) carrying 5 weight % (on a metal basis) of silverchloride having an average diameter of 50 nm was prepared. 1 g of theresulting first catalyst (silver-salt catalyst) was supported on acommercially available honeycomb-type cordieritc filter (diameter: 30mm, length: 12.6 mm, 400 cells/in²) by a wash-coating method, dried andthen heated to 600° C. stepwise to prepare an exhaust gas cleaner partcarrying the silver-salt catalyst.

Further, in the same manner as in Example 7, γ-alumina powder (specificsurface area: 200 m² /g) carrying 4 weight % (on a metal basis) of Ptwas prepared. 0.4 g of the resulting second catalyst (platinum catalyst)was supported on a honeycomb-type cordieritc filter (diameter: 30 mm,length: 6 mm), dried and then heated to 700° C. to prepare an exhaustgas cleaner part carrying the platinum catalyst.

With the silver-salt catalyst on the inlet side and the platinumcatalyst on the outlet side, these exhaust gas cleaner parts werecombined and charged in a reactor pipe. Under the same conditions as inExample 7, tests were conducted using the test gas having the samecomposition as in Table 6 except for containing diesel oil in an amountthree times that of nitrogen monoxide instead of propylene. The resultsare shown in FIG. 2.

Comparative Example 4

In the same manner as in Example 7, 5.4 g of an exhaust gas cleanercomposed of γ-alumina pellets carrying 5 weight % (on a metal basis) ofsilver chloride having an average diameter of 8 nm was charged in areactor pipe, and tests were conducted using the test gas having thecomposition shown in Table 6 under the same conditions as in Example 7.The results are shown in FIG. 2.

As is clear from FIG. 2, the exhaust gas cleaners in Examples 7 and 8can provide a high removal ratio of nitrogen oxides in a widetemperature range of the exhaust gas, while the exhaust gas cleaner inComparative Example 4 fails to do so.

EXAMPLE 9

The exhaust gas cleaner prepared in Example 7 was charged into a reactorpipe with the silver-salt catalyst on the inlet side and the platinumcatalyst on the outlet side.

Next, a test gas having the composition shown in Table 7 was caused topass through the reactor pipe at a rate of 4.4 liters per minute (normalstate), which corresponded to an overall apparent space velocity of30,000 h⁻¹ and catalyst ratios of the first and second catalysts ofabout 0.038 sec·g/ml and about 0.014 sec·g/ml, respectively. Thetemperature of the test gas was kept at 200° C. in the reactor pipe tocause a reaction between nitrogen monoxide and ethanol. The test wasrepeated while changing the temperature of the test gas as shown in FIG.3.

                  TABLE 7                                                         ______________________________________                                        Component       Concentration                                                 ______________________________________                                        Nitrogen monoxide                                                                             800 ppm                                                       Carbon monoxide 100 ppm                                                       Oxygen          10 volume %                                                   Ethanol         1600 ppm                                                      Nitrogen        Balance                                                       Water           10 volume % of the total volume                                               of the above components                                       ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 3.

EXAMPLE 10

The exhaust gas cleaner prepared in Example 8 was charged into a reactorpipe with the silver-salt catalyst on the inlet side and the platinumcatalyst on the outlet side. Under the same conditions (overall apparentspace velocity: 20,000 h⁻¹) as in Example 19, the tests were conductedusing the test gas having the composition shown in Table 7. The resultsare shown in FIG. 3.

Comparative Example 5

In the same manner as in Example 7, an exhaust gas cleaner comprisingγ-alumina pellets carrying 5 weight % (on a metal basis) of silverchloride having an average diameter of 2000 nm was prepared. 5.4 g ofthis exhaust gas cleaner was charged in a reactor pipe, and tests wereconducted using the test gas having the composition shown in Table 7under the same conditions as in Example 9. The results are shown in FIG.3.

EXAMPLE 11

In the same manner as in Example 7, 5 weight % (on a metal basis) ofsilver chloride was carried on commercially available γ-alumina pellets(diameter: 1.5 mm, length: 6 mm, specific surface area: 200 m² /g). Theresulting γ-alumina pellets were dried at 70° C. and calcined at 150°C., 200° C., 300° C., 400° C., 500° C., 600° C. stepwise to prepare afirst catalyst (silver-salt catalyst) carrying silver chloride having anaverage diameter of 200 nm.

Further, γ-alumina pellets (diameter: 1.5 mm, length: 6 mm, specificsurface area: 200 m² /g) were immersed in an aqueous solution ofchloroplatinic acid, dried and then heated to 700° C. to prepare asecond catalyst (platinum catalyst) carrying 3 weight % (on a metalbasis) of Pt.

An exhaust gas cleaner consisting of a mixture of 4 g of the firstcatalyst (silver-salt catalyst) and 2 g of the second catalyst (platinumcatalyst) was charged into a reactor pipe.

Next, a test gas having the composition shown in Table 8 was caused topass through the reactor pipe at a rate of 2.2 liters per minute (normalstate), which corresponded to an overall apparent space velocity of10,000 h⁻¹ and catalyst ratios of the first and second catalysts ofabout 0.1 sec·g/ml and about 0.05 sec·g/ml, respectively. Thetemperature of the test gas was kept at 200° C. in the reactor pipe tocause a reaction between nitrogen monoxide and liquefied petroleum gas.The test was repeated while changing the temperature of the test gas asshown in FIG. 4.

                  TABLE 8                                                         ______________________________________                                        Component        Concentration                                                ______________________________________                                        Nitrogen monoxide                                                                              800 ppm                                                      Oxygen           10 volume %                                                  Liquefied petroleum gas                                                                        1700 ppm                                                     Nitrogen         Balance                                                      Water            10 volume % of the total volume                                               of the above components                                      ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 4.

EXAMPLE 12

In the same manner as in Example 7, γ-alumina powder (average diameter:40 μm, specific surface area: 200 m² /g) carrying 5 weight % (on a metalbasis) of silver chloride having an average diameter of 300 nm wasprepared. 1 g of the resulting first catalyst (silver-salt catalyst) wassupported on a commercially available honeycomb-type cordieritc filter(diameter: 30 mm, length: 12.6 mm, 400 cells/in²) by a wash-coatingmethod, dried and then heated to 600° C. stepwise to prepare an exhaustgas cleaner part carrying the silver-salt catalyst.

Further, in the same manner as in Example 7, γ-alumina powder (averagediameter: 40 μm, specific surface area: 200 m² /g) carrying 3 weight %(on a metal basis) of Pt was prepared. 0.4 g of the resulting secondcatalyst (platinum catalyst) was supported on a honeycomb-typecordieritc filter (diameter: 30 mm, length: 6 mm), dried and then heatedto 700° C. to prepare an exhaust gas cleaner part carrying the platinumcatalyst.

With the silver-salt catalyst on the inlet side and the platinumcatalyst on the outlet side, these exhaust gas cleaner parts werecombined and charged in a reactor pipe. Under the same conditions (anoverall apparent space velocity of 10,000 h⁻¹)as in Example 11, testswere conducted using the test gas having the composition shown in Table8. The results are shown in FIG. 4.

EXAMPLE 13

Under the same conditions as in Example 11, tests were conducted usingthe exhaust gas cleaner prepared in Example 12 and the test gas havingthe same composition as in Table 8 except for containing propane insteadof liquefied petroleum gas.

Comparative Example 6

An exhaust gas cleaner comprising γ-alumina pellets carrying 5 weight %(on a metal basis) of silver chloride having an average diameter of 250nm was prepared in the same as in Example 11. 10 g of this exhaust gascleaner was charged in a reactor pipe, and tests were conducted usingthe test gas having the composition shown in Table 8 under the sameconditions as in Example 11. The results are shown in FIG. 4.

As is clear from FIGS. 3 and 4, the exhaust gas cleaners in Examples9-13 can provide high removal ratios of nitrogen oxides in a widetemperature range of the exhaust gas. On the other hand, the exhaust gascleaners in Comparative Examples 5 and 6 are effective only in a narrowtemperature range of the exhaust gas.

EXAMPLE 14

Commercially available y-alumina pellets (diameter: 1.5 mm, length: 6mm, specific surface area: 200 m² /g) were immersed in an aqueoussolution of silver nitrate (0.67 g of silver nitrate/20 ml of water) for20 minutes, dried at 80° C. for 2 hours, and then further dried at 180°C. for 2 hours in a dry nitrogen stream. After cooling to roomtemperature in a dry nitrogen stream, the γ-alumina pellets wereimmersed in an aqueous solution of ammonium chloride (0.5 g of ammoniumchloride/20 ml of water) for 12 hours to convert the silver nitrate onthe γ-alumina pellets into silver chloride. The γ-alumina pellets werethen dried at 80° C. for 2 hours in air, heated to 550° C. at a heatingrate of 2.5° C./min, and calcined at 550° C. for 5 hours in an nitrogenstream containing 10% of oxygen to prepare a first catalyst (silver-saltcatalyst) carrying 4 weight % (on a metal basis) of silver chloridehaving an average diameter of 200 nm.

Further, commercially available titania pellets (diameter: 1.5 mm,length: 6 mm, specific surface area: 20 m² /g) were immersed in anaqueous solution of chloroauric acid, dried and then heated to 700° C.to prepare a second catalyst (gold catalyst) carrying 1 weight % of Au.

An exhaust gas cleaner consisting of a mixture of 3.7 g of the firstcatalyst (silver-salt catalyst) and 1.8 g of the second catalyst (goldcatalyst) was charged into a reactor pipe.

Next, a test gas having the composition shown in Table 9 was caused topass through the reactor pipe at a rate of 4.4 liters per minute (normalstate), which corresponded to an overall apparent space velocity of20,000 h⁻¹ and catalyst ratios of the first and second catalysts of 0.05sec·g/ml and 0.025 sec·g/ml, respectively. The temperature of the testgas was kept at 250° C. in the reactor pipe to cause a reaction betweennitrogen monoxide and propylene in the test gas. The test was repeatedwhile changing the temperature of the test gas as shown in FIG. 5.

                  TABLE 9                                                         ______________________________________                                        Component       Concentration                                                 ______________________________________                                        Nitrogen monoxide                                                                             800 ppm                                                       Carbon monoxide 100 ppm                                                       Oxygen          10 volume %                                                   Propylene       1600 ppm                                                      Nitrogen        Balance                                                       Water           10 volume % of the total volume                                               of the above components                                       ______________________________________                                    

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 5.

EXAMPLE 15

With the test gas having the same composition as in Table 9 except forcontaining ethanol in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 14 were conductedusing the exhaust gas cleaner prepared in Example 14. The results areshown in FIG. 5.

EXAMPLE 16

In the same manner as in Example 14, γ-alumina powder (specific surfacearea: 200 m² /g) carrying 4 weight % (on a metal basis) of silverchloride having an average diameter of 200 nm was prepared. 1 g of theresulting first catalyst (silver-salt catalyst) was supported on acommercially available honeycomb-type cordieritc filter (diameter: 30mm, length: 12.6 mm, 400 cells/in²) by a wash-coating method, dried andthen heated to 600° C. stepwise to prepare an exhaust gas cleaner partcarrying the silver-salt catalyst.

Further, in the same manner as in Example 14, titania powder (specificsurface area: 50 m² /g) carrying 1 weight % (on a metal basis) of goldwas prepared. 0.4 g of the resulting second catalyst (gold catalyst) wascoated onto a honeycomb-type cordieritc filter (diameter: 30 mm, length:6 mm, 400 cells/in²), dried and then heated to 700° C. to prepare anexhaust gas cleaner part carrying the gold catalyst.

With the silver catalyst on the inlet side and the gold catalyst on theoutlet side, these exhaust gas cleaner parts were combined and chargedin a reactor pipe. Under the same conditions (overall apparent spacevelocity: 20,000 h⁻¹) as in Example 14, tests were conducted using thetest gas having the composition shown in Table 9. The results are shownin FIG. 5.

EXAMPLE 17

With the test gas having the same composition as in Table 9 except forcontaining kerosene in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 16 were conductedusing the exhaust gas cleaner prepared in Example 16. The results areshown in FIG. 5.

EXAMPLE 18

With the test gas having the same composition as in Table 9 except forcontaining ethanol in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 16 were conductedusing the exhaust gas cleaner prepared in Example 16. The results areshown in FIG. 5.

Comparative Example 7

An exhaust gas cleaner comprising γ-alumina pellets carrying 2 weight %(on a metal basis) of silver chloride having an average diameter of 180nm was prepared in the same manner as in Example 14. 3.6 g of thisexhaust gas cleaner was charged in a reactor pipe, and tests wereconducted using the test gas having the composition shown in Table 9under the same conditions as in Example 16. The results are shown inFIG. 5.

As is clear from FIG. 5, the exhaust gas cleaners in Examples 14-18 canprovide high removal ratios of nitrogen oxides in a wide temperaturerange of the exhaust gas. On the other hand, the exhaust gas cleaners inComparative Example 7 are effective only in a narrow temperature rangeof the exhaust gas.

EXAMPLE 19

In the same manner as in Example 7, 4 weight % (on a metal basis) ofsilver chloride having an average diameter of 250 nm was carried oncommercially available γ-alumina pellets (diameter: 1.5 mm, length: 6mm, specific surface area: 200 m² /g) to prepare a first catalyst(silver-salt catalyst).

Separately, commercially available titania pellets (diameter: 1.5 mm,length: 6 mm, specific surface area: 20 m² /g) were immersed in anaqueous solution of chloroauric acid, dried and then heated to 700° C.to prepare a second catalyst (gold catalyst) carrying 1 weight % of Au.

Further, commercially available γ-alumina pellets (diameter: 1.5 mm,length: 6 mm, specific surface area: 200 m² /g) were immersed in anaqueous solution of chloroplatinic acid, dried and then heated to 700°C. to prepare a third catalyst (platinum catalyst) carrying 2 weight %of Pt.

An exhaust gas cleaner consisting of a mixture of 3.7 g of the firstcatalyst (silver-salt catalyst), 1.8 g of the second catalyst (goldcatalyst) and 1.8 g of the third catalyst (platinum catalyst) wascharged into a reactor pipe.

Next, a test gas having the composition shown in Table 9 was caused topass through the reactor pipe at a rate of 4.4 liters per minute (normalstate), which corresponded to an overall apparent space velocity of15,000 h⁻¹ and catalyst ratios of the first, second and third catalystsof 0.05 sec·g/ml, 0.025 sec·g/ml and 0.025 sec·g/ml, respectively. Thetemperature of the test gas was kept at about 250° C. in the reactorpipe to cause a reaction between nitrogen monoxide and propylene in thetest gas. The test was repeated while changing the temperature of thetest gas as shown in FIG. 6.

The concentration of nitrogen oxides (sum of nitrogen monoxide andnitrogen dioxide) in the test gas after passing through the exhaust gascleaner was measured by a chemiluminescence analyzer to determine theremoval ratio of nitrogen oxides. The results are shown in FIG. 6.

EXAMPLE 20

With the test gas having the same composition as in Table 9 except forcontaining ethanol in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 19 were conductedusing the exhaust gas cleaner prepared in Example 19. The results areshown in FIG. 6.

EXAMPLE 21

In the same manner as in Example 19, γ-alumina powder (specific surfacearea: 200 m² /g) carrying 2 weight % (on a metal basis) of silverchloride having an average diameter of 180 nm was prepared. 1 g of theresulting first catalyst (silver-salt catalyst) was supported on acommercially available honeycomb-type cordieritc filter (diameter: 30mm, length: 12.6 mm, 400 cells/in²) by a wash-coating method, dried andthen heated to 600° C. stepwise to prepare an exhaust gas cleaner partcarrying the silver-salt catalyst.

Next, in the same manner as in Example 19, titania powder (specificsurface area: 50 m² /g) carrying 1 weight % (on a metal basis) of goldwas prepared. 0.4 g of the resulting second catalyst (gold catalyst) wascoated onto a honeycomb-type cordierire filter (diameter: 30 mm, length:6 mm, 400 cells/in²), dried and then heated to 700° C. to prepare anexhaust gas cleaner part carrying the gold catalyst.

Further, in the same manner as in Example 19, alumina powder (specificsurface area: 200 m² /g) carrying 1 weight % (on a metal basis) ofplatinum was prepared. 0.4 g of the resulting third catalyst (platinumcatalyst) was coated onto a honeycomb-type cordieritc filter (diameter:30 mm, length: 6 mm, 400 cells/in²), dried and then heated to 700° C. toprepare an exhaust gas cleaner part carrying the platinum catalyst.

With the silver-salt catalyst on the inlet side, the gold catalyst in anintermediate portion and the platinum catalyst on the outlet side, theseexhaust gas cleaner parts were combined and charged in a reactor pipe.Under the same conditions (an overall apparent space velocity of 15,000h⁻¹) as in Example 19, tests were conducted using the test gas havingthe composition shown in Table 9. The results are shown in FIG. 6.

EXAMPLE 22

With the test gas having the same composition as in Table 9 except forcontaining kerosene in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 21 were conductedusing the exhaust gas cleaner prepared in Example 21. The results areshown in FIG. 6.

EXAMPLE 23

With the test gas having the same composition as in Table 9 except forcontaining ethanol in an amount three times that of nitrogen monoxideinstead of propylene, the same tests as in Example 21 were conductedusing the exhaust gas cleaner prepared in Example 21. The results areshown in FIG. 6.

Comparative Example 8

An exhaust gas cleaner comprising γ-alumina pellets carrying 5 weight %(on a metal basis) of silver chloride having an average diameter of 250nm was prepared in the same manner as in Example 19. 3.6 g of thisexhaust gas cleaner was charged in a reactor pipe, and tests wereconducted using the test gas having the composition shown in Table 9under the same conditions as in Example 19. The results are shown inFIG. 6.

As is clear from FIG. 6, the exhaust gas cleaners in Examples 19-23 canprovide high removal ratios of nitrogen oxides in a wide temperaturerange of the exhaust gas. On the other hand, the exhaust gas cleaners inComparative Example 8 are effective only in a narrow temperature rangeof the exhaust gas.

As is described above in detail, by the exhaust gas cleaner according tothe present invention, nitrogen oxides, and optionally carbon monoxideand hydrocarbons can be efficiently removed from the exhaust gas havingan excess oxygen concentration in a wide temperature range. The exhaustgas cleaner and the method of the present invention are effective forremoving nitrogen oxides, and optionally carbon monoxide andhydrocarbons from exhaust gases such as those discharged from variouscombustors, automobile engines, etc.

What is claimed is:
 1. An exhaust gas cleaner for removing nitrogenoxides from an exhaust gas containing nitrogen oxides and oxygen in anamount larger than its stoichiometric amount relative to unburnedcomponents in said exhaust gas, which consists essentially ofa firstcatalyst on an inlet side of said exhaust gas cleaner and a secondcatalyst on an outlet side of said exhaust gas cleaner, said firstcatalyst consisting essentially of 0.2-15 weight % (on a metal basis) ofat least one silver salt supported on a porous inorganic oxide, and saidsecond catalyst consisting essentially of (a) 0.2-15 weight % (on ametal basis) of at least one silver salt and (b) up to 2 weight % (on ametal basis) of copper or copper oxide supported on a porous inorganicoxide, said silver salt for each catalyst being selected from the groupconsisting of silver halides silver sulfate and silver phosphate.
 2. Theexhaust gas cleaner for removing nitrogen oxides according to claim 1,wherein said exhaust gas cleaner further comprises a three-dimensionalstructure of ceramics or metal on which said first catalyst and saidsecond catalyst are coated.
 3. The exhaust gas cleaner for removingnitrogen oxides according to claim 1, wherein said porous inorganicoxide for supporting said first and second catalysts is in the form ofpellet or granule.
 4. The exhaust gas cleaner for removing nitrogenoxides according to claim 1, wherein said porous inorganic oxide is atleast one ceramic selected from the group consisting of alumina andcomposite oxides thereof.
 5. An exhaust gas cleaner for cleaning anexhaust gas containing nitrogen oxides, unburned components comprisingcarbon monoxide and hydrocarbons, and oxygen in a amount larger than itsstoichiometric amount relative to said unburned components, by removingnitrogen oxides by reduction and carbon monoxide and hydrocarbons byoxidation from said exhaust gas, which consists essentially offirst,second and third catalysts in this order from an inlet side to an outletside of said exhaust gas cleaner, said first catalyst consistingessentially of 0.2-15 weight % (on a metal basis) of at least one silversalt supported on a porous inorganic oxide, said second catalystconsisting essentially of (a) 0.2-15 weight % (on a metal basis) of atleast one silver salt and (b) up to 2 weight % (on a metal basis) ofcopper or copper oxide supported on a porous inorganic oxide, and saidthird catalyst consisting essentially of up to 2 weight % (on a metalbasis) of at least one element selected from the group consisting of Pt,Pd, Ru, Rh, Ir and Au supported on a porous inorganic oxide, said silversalt for each catalyst being selected from the group consisting ofsilver halides, silver sulfate and silver phosphate.
 6. The exhaust gascleaner for cleaning an exhaust gas according to claim 5, wherein saidexhaust gas cleaner further comprises a three-dimensional structure ofceramics or metal on which at least one of said first, second and thirdcatalysts is coated.
 7. The exhaust gas cleaner for cleaning an exhaustgas according to claim 5, wherein said porous inorganic oxide supportingsaid first, second and third catalysts is in the form of pellet orgranule.
 8. The exhaust gas cleaner for cleaning an exhaust gasaccording claim 5, wherein said porous inorganic oxide is γ-alumina orcomposite oxides thereof for said first and second catalysts, and atleast one ceramic selected from the group consisting of alumina,titania, zirconia and composite oxides thereof for said third catalyst.9. An exhaust gas cleaner for removing nitrogen oxides from an exhaustgas containing nitrogen oxides and oxygen in an amount larger than itsstoichiometric amount relative to unburned components in said exhaustgas, which consists essentially ofa first catalyst consistingessentially of 0.2-15 weight % (on a metal basis) of at least one silversalt having an average diameter of 10-1000 nm and supported on a porousinorganic oxide, said silver salt being selected from the groupconsisting of silver halides, silver sulfate and silver phosphate, and asecond catalyst consisting essentially of 0.02-5 weight % (on a metalbasis) of at least one element selected from the group consisting of Pt,Pd, Ru, Rh, Ir and Au supported on a porous inorganic oxide.
 10. Theexhaust gas cleaner for removing nitrogen oxides according to claim 9,wherein said second catalyst comprises a gold catalyst comprising Ausupported on a porous inorganic oxide and a platinum catalyst comprisingat least one element selected from the group consisting of Pt, Pd, Ru,Rh and Ir supported on a porous inorganic oxide.
 11. The exhaust gascleaner for removing nitrogen oxides according to claim 9, wherein saidfirst catalyst and said second catalyst are respectively disposed on aninlet side and an outlet side of said exhaust gas cleaner.
 12. Theexhaust gas cleaner for removing nitrogen oxides according to claim 11,wherein said second catalyst comprises a gold catalyst comprising Ausupported on a porous inorganic oxide and a platinum catalyst comprisingat least one element selected from the group consisting of Pt, Pd, Ru,Rh and Ir supported on a porous inorganic oxide, said gold catalystbeing disposed in an intermediate portion of said exhaust gas cleanerand said platinum catalyst being disposed on an outlet side of saidexhaust gas cleaner.
 13. The exhaust gas cleaner for removing nitrogenoxides according to claim 9, wherein said exhaust gas cleaner furthercomprises a three-dimensional structure of ceramics or metal on whichsaid first catalyst and said second catalyst are coated.
 14. The exhaustgas cleaner for removing nitrogen oxides according to claim 9, whereinsaid porous inorganic oxide for supporting said first and secondcatalysts is in the form of pellet or granule.
 15. The exhaust gascleaner for removing nitrogen oxides according to claim 9, wherein saidporous inorganic oxide for said first catalyst is alumina or a compositeoxide thereof, and said porous inorganic oxide for said second catalystis at least one ceramic selected from the group consisting of alumina,titania, zirconia and composite oxides thereof.
 16. The exhaust gascleaner for removing nitrogen oxides according to claim 10, wherein saidporous inorganic oxide for said gold catalyst is at least one ceramicselected from the group consisting of alumina, titania, zinc oxide,magnesium oxide and composite oxides thereof.
 17. The exhaust gascleaner for removing nitrogen oxides according to claim 15, wherein saidporous inorganic oxide for said gold catalyst is at least one ceramicselected from the group consisting of alumina, titania, zinc oxide,magnesium oxide and composite oxides thereof.